I have some update: Today the second pair of KH 120 arrived and the last 3 hours I measured all of them at the same position. Slight variations in the mic position are possible of course and I slighty varied the mic between measurements. Volume was around 84 dB at 1 m distance and I measured at ~10 cm in front of the tweeter and at 1 m distance.
Fig 1 shows measurements at the tweeter directly. The green curve is my old right speaker that showed that peak at 15.5 kHz, the blue is my old left speaker that didn't show that peak or at least less pronounced. Green and black curves are the new monitors (green = new 1, black = new 2). The "new 2" monitor is much more (!) noisy when connected to the interface compared to all three other monitors and that's why I will not keep it. Maybe a grounding issue? I tend to keep green and red, cause they both show that resonance with a similar amplitude and I think that's normal as three out of four speaker show it. What do you think? I also attached the mdat files below in case some wants to have a look.
Fig 1
Fig. 2: at 1 m distance:
The old left speaker has a small peak in the distortion measurement at ca. 470 Hz (see Fig 3) which is not present in all others (e.g. monitor new 1, Fig 4)
Fig 3:
Fig 4
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